[PATCH v3 06/33] util/async: aio_co_schedule(): support reschedule in same ctx

[Vladimir Sementsov-Ogievskiy]

With the following patch we want to call wake coroutine from thread.
And it doesn't work with aio_co_wake:
Assume we have no iothreads.
Assume we have a coroutine A, which waits in the yield point for
external aio_co_wake(), and no progress can be done until it happen.
Main thread is in blocking aio_poll() (for example, in blk_read()).

Now, in a separate thread we do aio_co_wake(). It calls  aio_co_enter(),
which goes through last "else" branch and do aio_context_acquire(ctx).

Now we have a deadlock, as aio_poll() will not release the context lock
until some progress is done, and progress can't be done until
aio_co_wake() wake the coroutine A. And it can't because it wait for
aio_context_acquire().

Still, aio_co_schedule() works well in parallel with blocking
aio_poll(). So we want use it. Let's add a possibility of rescheduling
coroutine in same ctx where it was yield'ed.

Fetch co->ctx in same way as it is done in aio_co_wake().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/block/aio.h | 2 +-
 util/async.c        | 8 ++++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/block/aio.h b/include/block/aio.h
index 5f342267d5..744b695525 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -643,7 +643,7 @@ static inline bool aio_node_check(AioContext *ctx, bool 
is_external)
 
 /**
  * aio_co_schedule:
- * @ctx: the aio context
+ * @ctx: the aio context, if NULL, the current ctx of @co will be used.
  * @co: the coroutine
  *
  * Start a coroutine on a remote AioContext.
diff --git a/util/async.c b/util/async.c
index 674dbefb7c..750be555c6 100644
--- a/util/async.c
+++ b/util/async.c
@@ -545,6 +545,14 @@ fail:
 
 void aio_co_schedule(AioContext *ctx, Coroutine *co)
 {
+    if (!ctx) {
+        /*
+         * Read coroutine before co->ctx.  Matches smp_wmb in
+         * qemu_coroutine_enter.
+         */
+        smp_read_barrier_depends();
+        ctx = qatomic_read(&co->ctx);
+    }
     trace_aio_co_schedule(ctx, co);
     const char *scheduled = qatomic_cmpxchg(&co->scheduled, NULL,
                                            __func__);
-- 
2.29.2